The present invention relates in general to semiconductor switching devices, and in particular to a more efficient implementation of a bilateral load switch that exhibits lower resistance.
There are many electronic applications that require high-current switches for connecting or disconnecting a power source (e.g., a battery) from a load. Portable electronic devices, for example, have rechargeable batteries that connect to a charger by such a switch. To protect the battery from overcharging and from damage due to excessive current, current flow from the charger is monitored to detect when the battery has been fully charge. This is typically accomplished by sensing the voltage drop across a small value resistor in series with the switch and feeding this information back to a microcontroller to determine the charge state. A very good switch, having good off-state isolation and minimum on-state resistance, is needed to disconnect the charger from the battery once a fully charged state is detected. P-channel power field-effect transistors (MOSFETs) are often used as the switching component because of their exceptionally low on-state source-drain resistance (RDSON), and their ability to interface easily to the control circuitry.
Conventional power MOSFETs have an intrinsic diode element that allows damaging current to flow if the battery or the charger is connected in reverse polarity. For this reason, it is known to connect a pair of p-channel MOSFETs so that their diodes oppose each other to prevent reverse connection damage. FIG. 1 shows a switch made up of two p-channel MOSFETs MP1 and MP2 connected in a common-source configuration with their intrinsic diodes 100 and 102 opposing each other. The switch circuit also includes an n-channel MOSFET MN1 that drives the gate terminals of transistors MP1 and MP2. Turning on MN1 pulls the gate terminals of MP1 and MP2 down to ground and biases the p-channels into an on (i.e., conductive) state. When turned on and conducting, current can flow in either direction between the two terminals IN and OUT. A fuse element 104 is commonly inserted along this current path to protect against over-current conditions. A series resistor Rs is also provided along this current path, a magnitude of a voltage drop across which is sensed by external circuitry and used to indicate the level of charge transfer. Resistors R1 and R2 are used to control the slew rate of transistors MP1 and MP2 when turning on (R2) and turning off (R1).
Existing implementations of this type of switch provide two independent transistors, and the remaining components external to the chip. A typical example is shown in FIG. 2 where to reduce RDSON, the source electrode of each one of the two transistors MP1 and MP2 has been provided with four leads 200, 202, 204 and 206. Each lead connects to the internal source electrode of a transistor via multiple (e.g., 4) bond wires 208, to further reduce the on resistance of the switch. A number of other leads are provided to facilitate external connections to the fuse, Rs, slew control resistors and control and power pins. This arrangement not only results in a device with a larger pin count, it requires numerous bond wires (e.g., 8 wires for each source connection) in addition to the various external components (fuse element, sense resistor Rs, and slew rate control resistors R1 and R2).
The present invention offers a novel implementation of a high current bilateral switch that greatly simplifies the component requirements while at the same time simplifying the construction of the power transistors. Broadly, the bilateral switch of the present invention minimizes the on-state resistance by making a common-source connection between the switch transistors internal to the package. Wire bonds internally connecting the source electrodes of the transistors also provide the function of one or both of a current sense resistor and a fuse element. In a preferred embodiment, the same element that is used to implement the internal sense resistor also acts as the fuse element. The resistance and fusing current of the sense resistor implemented according to this embodiment can be adjusted by the type of material used, the dimensions and the number of the bonding wires. The resulting switch requires fewer external components, fewer pins and is more economical to manufacture.
Accordingly, in one embodiment, the present invention provides a semiconductor switch housed inside a package, the switch including a first transistor having a source electrode, a drain electrode and a gate electrode; a second transistor having a source electrode, a drain electrode and a gate electrode; and a common-source bond wire electrically connecting the source electrode of the first transistor directly to the source electrode of the second transistor. The switch further includes a first source bond wire connecting the source electrode of the first transistor to a first external connector, and a second source bond wire connecting the source electrode of the second transistor to a second external connector, wherein, the common-source bond wire acts as an internal sense resistor. In another embodiment, the common-source bond wire also acts as a fuse element.
In yet another embodiment, the present invention provides a bilateral semiconductor switch inside a package, the switch including a first power transistor having a source electrode, a drain electrode coupled to a first I/O terminal and a gate electrode; a second transistor having a source electrode, a drain electrode coupled to a second I/O terminal, and a gate electrode; a sense resistor connecting the source electrode of the first transistor to the source electrode of the second transistor; and a control transistor having a gate electrode coupled to receive a control signal, a source electrode coupled to receive a low signal level, and a drain electrode adapted to be externally coupled to be the gate electrodes of the first and the second transistors. The switch further includes a fuse element connecting in series between the sense resistor and the source electrode of one of the first or second transistors.
A better understanding of the nature and advantages of the low resistance bilateral load switch of the present invention may be gained with reference to the detailed description and drawings below.